Hadoop 2.0 enables clusters to grow as large as 4000 nodes within deployments that contain multiple clusters. I think that companies like Google and Facebook each run tens of thousands of nodes.
Using Yarn, developers can run additional applications within the cluster by monitoring what the applications need, and then creating CPU/RAM containers within the cluster (and across clusters?) to run them.
There’s speculation that eventually Yarn could provide a PaaS using Hadoop in order to compete with VMWare’s Cloud Foundry. I suppose that while with VMWare you need to first think in terms of virtualizing hardware components and an operating system, Yarn jumps past that to provide an environment that’s abstracted for a specific application.